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  • Thread starter lc99
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Homework Statement


upload_2018-4-1_22-22-18.png


Homework Equations




The Attempt at a Solution


Why is this wrong? --
P1 = 1000 (1% of atm)
P1 = P2[/B]

99000 = 101300 + pgh
99000-101300 = pgh
solving for h, h = 0.1 m
 

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Answers and Replies

  • #2
Merlin3189
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What height does your h represent?
And what height are they asking for?
 
  • #3
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t
What height does your h represent?
And what height are they asking for?
hey are asking for the height the water rises as a result of the reduce in pressure. the height i gave would be the change in height
 
  • #4
Merlin3189
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(Sorry, I've been out.)

Yes, you know what they are asking, but " the height i gave would be the change in height" Change in what height?
They are asking for the change in level of the right hand tube.

I nearly suggested you draw a diagram, but thought we could manage without. Now I think we need a diagram, as I still don't understand what you are calculating.

It would help if you explained where your numbers came from and how you produced your equations.

What are P1 and P2 which are apparently the same?

Where does 101300 come from and what does it represent? I can guess where the 99000 comes from and what it's unit is.
 
  • #5
161
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(Sorry, I've been out.)

Yes, you know what they are asking, but " the height i gave would be the change in height" Change in what height?
They are asking for the change in level of the right hand tube.

I nearly suggested you draw a diagram, but thought we could manage without. Now I think we need a diagram, as I still don't understand what you are calculating.

It would help if you explained where your numbers came from and how you produced your equations.

What are P1 and P2 which are apparently the same?

Where does 101300 come from and what does it represent? I can guess where the 99000 comes from and what it's unit is.
upload_2018-4-2_12-19-23.png
 

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  • #6
Merlin3189
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I shouldn't have asked! But I think it might help a bit. At least you can see where you went wrong now.

Δh does appear to be the amount the water rises in the right hand tube.

All you need now is the calculation of the pressure difference.

Edit: So what measurements do you use to calculate the pressure difference across the manometer?
 
Last edited:
  • #7
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I shouldn't have asked! But I think it might help a bit. At least you can see where you went wrong now.

Δh does appear to be the amount the water rises in the right hand tube.

All you need now is the calculation of the pressure difference.

Edit: So what measurements do you use to calculate the pressure difference across the manometer?
I shouldn't have asked! But I think it might help a bit. At least you can see where you went wrong now.

Δh does appear to be the amount the water rises in the right hand tube.

All you need now is the calculation of the pressure difference.

Edit: So what measurements do you use to calculate the pressure difference across the manometer?
Pressure difference in abs(P2-P1) = p*g(y2-y1)?
P2 = 99000, y2 = h
P1 = 100000, y1= -h
Ahhhh
i think i got it.
1000= pg(h-(-h))
= 2pgh
h = 0.05 m or 5 cm.

I didn't think that y1 would be -h.
 
  • #8
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do we just take the absolute value of p2-p1? So that i dont get a negative value?
 
  • #9
Merlin3189
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I still struggle to follow your working when you use symbols not on your diagram nor in the statement of the problem, but it looks as if you are right now.

The point was that the pressure difference is related to the height difference, not simply to the change.

do we just take the absolute value of p2-p1? So that i dont get a negative value?
Since I still don't know what you mean by P! and P2, I shouldn't say! But IMO you can do either.

If you are careful, then you will get the right result using actual values. If you use simple magnitudes, you will get the right results in simple cases, probably more easily.
Here you could simply say the difference in pressure is 1% atm = 1000 Pa
So hρg = 1000 , so h=10.2cm meaning the difference in height between the tubes (irrespective of their diameter or orientation)

This seems to be where you were first in error - the difference of height is not the same as the change in height.
 
  • #10
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I still struggle to follow your working when you use symbols not on your diagram nor in the statement of the problem, but it looks as if you are right now.

The point was that the pressure difference is related to the height difference, not simply to the change.


Since I still don't know what you mean by P! and P2, I shouldn't say! But IMO you can do either.

If you are careful, then you will get the right result using actual values. If you use simple magnitudes, you will get the right results in simple cases, probably more easily.
Here you could simply say the difference in pressure is 1% atm = 1000 Pa
So hρg = 1000 , so h=10.2cm meaning the difference in height between the tubes (irrespective of their diameter or orientation)

This seems to be where you were first in error - the difference of height is not the same as the change in height.
Ahh. I get it now. I was not really aware that the difference in height isn't the same as change in height of water. Thank you
 
  • #11
Merlin3189
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U-tube.png

though here, h=change in height and d is difference in height
 

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